1. Field of the Invention
This invention pertains to a method and apparatus for thermally developing a photosensitive element, and particularly to a method and apparatus for supporting a development medium with a non-rotating surface to provide contact with the photosensitive element.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution, e.g., solvent or aqueous-based washout, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing. However, developing systems that treat the element with a solution are time consuming since drying for an extended period (0.5 to 24 hours) is necessary to remove absorbed developer solution.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. Nos. 3,060,023 (Burg et al.); 3,264,103 (Cohen et al.); 5,015,556 (Martens); 5,175,072 (Martens); 5,215,859 (Martens); and 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated/removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
Processors for thermal development of flexographic printing elements are known. U.S. Pat. No. 5,279,697 describes an automated process and apparatuses for handling an irradiated printing element and accomplishing heating and pressing to remove the unirradiated composition from the element. One embodiment of the thermal development apparatus shown in FIGS. 15 and 16 includes a heated plate that is brought into intimate contact with an absorbent material which in turn is in contact with an upper surface of a flexible sheet that resides on a base. The heated plate remains in place for a sufficient time to liquefy a portion of polymer material in the sheet and allow the liquified polymer material to be absorbed onto the absorbent material. The heated plate is removed and the flexible sheet and the absorbent material advance together while rolls separate the absorbent material from the formed flexographic sheet.
U.S. Pat. No. 5,279,697 describes another embodiment of an automated process and apparatus for handling an irradiated printing element and accomplishing repeated heating and pressing to remove the unirradiated composition from the element. WO 2001/18604 also describes a method and apparatus for thermal processing a photosensitive element. In both thermal processing apparatuses the absorbent material is a continuous sheet of a web, typically a non-woven, which is passed over a hot roll. The hot roll is urged towards a drum carrying the photosensitive element pressing the web against the photosensitive element and forming a nip. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent web. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then separated from the element.
A problem sometimes arises in thermal development processors in which the rotating hot roller brings the absorbent material into contact with the photosensitive element. Existing thermal development processes do not always adequately clean out or remove the uncured photopolymer from the recessed areas. In order to achieve improved relief uniformity the pressure at the nip between the hot roll carrying the absorbent material and drum carrying the photosensitive element can be increased to impress the absorbent material into the recessed areas. Increased nip pressure compresses the element creating a wider contacting zone for the nip, i.e., footprint, on the element along an axial length of the drum as well as increases the residence (i.e., dwell) time for the transfer of heat to the element. However, increased residence time that raises temperatures at or above the glass transition temperature of the base support for the element, can result in distortion of the resulting printing form or plate. In addition the hot roll has a tendency to deflect or bow along its length, resulting in non-uniform application of pressure to the photosensitive element along axial length of the contact zone. Non-uniform application of pressure can contribute to the resulting relief structure of the printing form having non-uniform relief depth.
Relief printing forms having distortion/s in the support and/or the cured photopolymeric layer result in poor print performance. In multicolor printing, when one or more of the relief printing forms have distortion the printed image has poor registration. Even in single color printing, distortion in the relief printing form may print an image that is not an accurate reproduction of its original, so called image infidelity, by printing straight lines as curves for example. The relief printing form having distortion/s may also incompletely print the image due to intermittent contact of the inked surface of the printing form to the printed substrate.
In addition, the existing thermal development processors can be expensive and difficult to manufacture as well as maintain, due to the mechanical complexity of the rotating and moveable hot roller and the need to monitor temperature with sensors on the moving parts.